Carrier and Light Trapping in Graded Quantum Well Laser Structures

We investigated the carrier and light trapping in GaInAs/AlGaAs single quantum well laser structures by means of time resolved photoluminescence and Raman spectroscopy. The influence of the shape and depth of the confinement potential and of the cavity geometry was studied by using different AlGaAs/GaAs short-period superlattices as barriers. Our results show that grading the optical cavity improves considerably both carrier and light trapping in the quantum well, and that the trapping efficiency is enhanced by increasing the graded confining potential.Comment: PDF-format, 15 pages (including 4 figures), Applied Physics Letters (June 2000

Laser crystallization and structuring of amorphous germanium

The short-pulse laser crystallization and interference structuring of amorphous germanium films were investigated by time resolved reflection measurements and Raman spectroscopy, We demonstrate that submicrometer crystalline structures with very sharp lateral interfaces can be produced by laser interference crystallization of nonhydrogenated samples. In hydrogenated films, on the other hand, the film surface disrupts upon laser exposure leading to the formation of a free-standing crystalline membrane, The Raman spectra of laser crystallized germanium display effects of finite crystallite size and stress. (C) 1997 American Institute of Physics.70263570357

Polarization Control of the Non-linear Emission on Semiconductor Microcavities

The degree of circular polarization ($\wp$) of the non-linear emission in semiconductor microcavities is controlled by changing the exciton-cavity detuning. The polariton relaxation towards \textbf{K} $\sim 0$ cavity-like states is governed by final-state stimulated scattering. The helicity of the emission is selected due to the lifting of the degeneracy of the $\pm 1$ spin levels at \textbf{K} $\sim 0$. At short times after a pulsed excitation $\wp$ reaches very large values, either positive or negative, as a result of stimulated scattering to the spin level of lowest energy ($+1/-1$ spin for positive/negative detuning).Comment: 8 pages, 3 eps figures, RevTeX, Physical Review Letters (accepted

Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells

We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of $\sim 4$ meV to zero, at a field of $\sim$35 kV/cm. This decrease, due to the field-induced deformation of the exciton wavefunction, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separation. However, a new theory that considers the modification of screening with that separation is needed to account for the observed dependence on excitation power of the individual energies of the two exciton components.Comment: 5 pages, 4 eps figures, RevTeX, Physical Review Letters (in press

Spin relaxation in low-dimensional systems

We review some of the newest findings on the spin dynamics of carriers and excitons in GaAs/GaAlAs quantum wells. In intrinsic wells, where the optical properties are dominated by excitonic effects, we show that exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors. In doped wells, the two spin components of an optically created two-dimensional electron gas are well described by Fermi-Dirac distributions with a common temperature but different chemical potentials. The rate of the spin depolarization of the electron gas is found to be independent of the mean electron kinetic energy but accelerated by thermal spreading of the carriers.Comment: 1 PDF file, 13 eps figures, Proceedings of the 1998 International Workshop on Nanophysics and Electronics (NPE-98)- Lecce (Italy

Dispersion mapping of spin-dependent polariton dynamics in CdTe microcavities

International audienc

Short-pulse laser-induced crystallization of intrinsic and hydrogenated amorphous germanium thin films

We report on the laser crystallization of intrinsic (a-Ge) and hydrogenated (a-Ge:H) amorphous germanium thin films using short, i.e., ns range, laser pulses. The influence of hydrogen on the phase transitions was investigated by monitoring the reflectance of the sample during laser irradiation. We determined the thresholds for melting (36 mJ/cm(2)) and for surface damage (66 mJ/cm(2)) of the a-Ge film. In a-Ge:H, hydrogen effuses on a short time scale (10 ns) upon laser irradiation. The effusion leads to the formation of a lifted-off (100 nm thick) crystalline Ge membrane, leaving behind a rough and incompletely crystallized surface. In a-Ge, on the other hand, no surface disruption is observed. The Raman spectra of hydrogenated samples are dominated by stress effects, while those corresponding to non-hydrogenated samples are dominated by crystallite size distribution effects. We also conclude that laser-induced annealing, carried out by applying several pulses with increasing intensity, can be used as a tool for the crystallization of a-Ge:H samples without hydrogen-induced surface damage. (C) 1997 American Institute of Physics.82105159516

Detuning dependence of polariton spin dynamics

International audienc

Growth Mechanisms In Laser Crystallization And Laser Interference Crystallization

The processes involved in the pulsed laser crystallization of amorphous silicon thin films were studied using transient reflection measurements. A model of the melting and solidification induced by the laser exposure, based on a one-dimensional calculation of the heat flow, was used to simulate the time-dependent reflectivity, yielding agreement with the experiments. Two laser beams interfering on the sample surface lead to the growth of long grains (up to 1.5 μm), with a well-defined orientation. We conclude that this lateral growth results from explosive crystallization combined with liquid phase growth. © 1998 Elsevier Science B.V. All rights reserved.227-230PART 2921924Sameshima, T., Usui, S., (1986) Mater. Res. Soc. Symp. Proc., 71, p. 435Sposili, R.S., Im, J.S., (1996) Appl. Phys. Lett., 69, p. 2864Kim, H.J., Im, J.S., (1995) Appl. Phys. Lett., 68, p. 1513Wood, R.F., Geist, G.A., (1986) Phys. Rev. B., 34, p. 2606Heintze, M., Santos, P.V., Nebel, C.E., Stutzmann, M., (1994) Appl. Phys. Lett., 64, p. 3148Thomson, M.O., Galvin, G.J., Mayer, J.W., Peercy, P.S., Poate, J.M., Jacobson, D.C., Cullis, A.G., Chew, N.G., (1984) Phys. Rev. Lett., 52, p. 2360Toet, D., Aichmayr, G., Mulato, M., Santos, P.V., Spangenberg, A., Bergmann, R.B., (1997) Mater. Res. Soc. Symp. Proc., 467, p. 337Kim, H.J., Im, J.S., (1996) Mater. Res. Soc. Symp. Proc., 397, p. 40

Preservation of quantum coherence after exciton-exciton interaction in quantum wells

The dynamics of exciton-exciton interaction in quantum wells has been investigated by monitoring the time-resolved resonant secondary emission that follows excitation with linearly and circularly polarized light. Preservation of quantum beating in the cross-polarized emission demonstrates that spin relaxation can take place, for some scattering channels, without total quantum coherence loss. Interexciton electron exchange is the scattering mechanism that explains the persistence of the beating and, since it is sensitive to the fine structure of excitons, the shift by pi in the phase of the beating observed in the experiment